Electroconductive, corrosion resistant high silicon alloy

ABSTRACT

Disclosed is a silicon base alloy containing sufficient dopant to provide an electrical conductivity in excess of 100 (ohmcentimeters) 1, and sufficient transition group metal to provide a volumetric coefficient of expansion upon solidification of less than about 10 percent, and balance silicon.

United States Patent Related US. Application Data US. Cl 75/134 S,204/293 Int. Cl. B01k 3/06, COld 1/08, COlb 11/25 Field of Search 75/134S, 122, 123 L;

References Cited UNITED STATES PATENTS 1/1970 Bianchi et a1 204/293 XHoek'e Dec. 17 1974 .l 9

[54] fi g iggfigg iig g ffifg FOREIGN PATENTS OR APPLICATIONS 7,003,7749/1970 Netherlands 204/292 [75] Inventor: Howard H. Hoekje, Akron, Ohio[73] Assignee: PPG Industries, Inc., Pittsburgh, Pa. OTHER PUBLICATIONSFiledi g- 1973 Effect of Boron on the Resistivity and Rectification [21]AppL No 391,118 Characteristics of Silicon," Metal Transactions, Vol.

185, June 1949, Pp- 385-388.

Primary ExaminerL. Dewayne Rutledge Assistant Examiner-Arthur J. SteinerAttorney, Agent, orFirm-Richard M. Goldman [5 7] ABSTRACT Disclosed is asilicon base alloy containing sufficient dopant to provide an electricalconductivity in excess of 100 (ohm-centimeters), and sufficienttransition group metal to provide a volumetric coefficient of expansionupon solidification of less than about 10 percent. and balance silicon.

16 Claims, 5 Drawing Figures PATENTED 5531 71974 3,854, 940

SHEET 1 [1F 5 PATENTEI] DEC] 7 I974 SHEET 2 BF 5 FIG PATENTED mm 7 m4SHEET 5 OF 5 FIG ELECTROCONDUCTIVE, CORROSION RESISTANT HIGH SILICONALLOY CROSS-REFERENCE TO RELATED APPLICATIONS This is acontinuation-in-part of my commonly assigned copending U.S. ApplicationSer. No. 260,790 filed June 8, 1972, of my commonly assigned copendingU.S. Application Ser. No. 336,288 filed Feb. 27, 1973 which is in turn acontinuation-in-part of said U.S. Application Ser. No. 260,790, and ofmy commonly assigned co-pending U.S. Application Ser. No. 356,972 filedMay 3, 1973, which is in turn a continuation-in-part of said U.S.Application Ser. No. 336,288 and of said U.S. Application Ser. No.260,790.

BACKGROUND OF THE INVENTION Elemental silicon and alloys containinglarge amounts of elemental silicon have been found to provide aparticularly outstanding electroconductive material for use in acidicmedia, such as anodes for electrolytic cells. Such anodes areparticularly described in my commonly assigned, co-pending applicationsfor Electrodes Having Silicon Base Members" filed June 8, 1972, Ser. No.260,790. Such silicon anodes, while having satisfactory electrochemicalproperties, have been difficult to cast, and have been characterized bya higher degree of brittleness, i.e., a lower impact strength, than isdesired for such service.

SUMMARY It has surprisingly now been found that a particularlysatisfactory electrode may be provided by a silicon alloy containingsufficient dopant to provide an electrical conductivity of greater thanabout 100 (ohmcentimeters) and sufficient transition metal to provide avolumetric expansion coefficient, upon solidification, of less than theplus percent expansion characteristic of silicon itself. Such alloys arecharacterized by a predominant, discontinuous, silicon-rich phase,substantially continuous rivulets of a transition metalrich phasesurrounding the silicon rich phase and forming the boundaries thereof,and discrete nodules of a dopant-rich phase.

DESCRIPTION According to this invention, en electroconductive,corrosion-resistance, castable, high silicon alloy is provided. Thealloy is particularly useful for providing electroconductive elementsfor use in acidic media, e.g., as anodes in electrolytic cells such aschlorine cells. Such anodes are particularly described in my commonlyassigned, co-pending applications Ser. No. 260,790, filed June 8, 1972for Electrodes Having Silicon Base Members and Ser. No. 356,972, filedMay 3, 1973, for Electrolytic Cell for the Electrolysis of alkali MetalChlorides Having Bipolar Electrodes."

The silicon alloy described herein has an electroconductivity greaterthan 100 (ohm-centimeter), and preferably in excess in 1,000 or more(ohmcentimeter), and frequently as high as 1,500 (ohmcentimeter). Thealloy is further characterized by its resistance to corrosion,particularly in acidic media,

such as in acidified brine solutions where nascent chlo- I rine isevolved.

The silicon alloy described herein is characterized by its readycastability. Elemental silicon alloys having particularly low contentsof alloying elements are characterized by expansion upon solidification.The coefficient of expansion of elemental upon solidification, i.e.,AV/Vo where V0 is the initial volume, and AV is the volumetric expansionin consistent units, is on the order of about 10 percent. This expansionupon solidification sets up internal stresses and thermal stresses whichcan result in the failure of the casting. The silicon alloy of thisinvention is characterized by the substantial absence of such stresses.ln one exemplification, the silicon alloy of this invention has areduced co-efficient of expansion upon solidification, for example +5percent or less.

On an elemental basis, the alloy of this invention contains silicon, atransition metal, and a dopant. The dopant and the transition metal aregenerally present as silicides and solid solutions with silicon.

The dopant is generally nitrogen, phosphorous, aluminum, or boron. Mostcommonly boron or phosphorous is the dopant. Boron is the preferreddopant. The amount of dopant is sufficient to provide anelectroconductivity greater than (ohm-centimeter). This is on the orderof about 0.2 weight percent or more.

Lesser amounts of dopant, e.g., less than about 0.2 weight percent,while beneficial in increasing the conductivity of the alloy, generallydo not raise the conductivitiy to above 100 (ohm-centimeter). As apractical matter, the concentration of the dopant will be greater thanthe solubility of the dopant in molten silicon. This is generally on theorder of about 0.5 weight percent. The concentration of dopant will,however, not be great enough-to increase the susceptibility of the alloyto corrosion or spalling. As a practical matter, this is on the order ofabout 2 weight percent,

Whenever a transition metal rich phase or transition metal siliciderich'phase, is referred to herein, it is to be understood that the phasereferred to is the phase in the alloy having the higher or highestcontent of transition metal either as the metal or as a silicidethereof, expressed on an elemental basis. The transition metal richphase generally contains twenty or more atomic percent of the transitionmetal, elemental basis, most frequently as the silicide.

The transition metal will generally be present as a silicide of thetransition metal such as manganese disilicide, chromium disilicide, irondisilicide, cobalt trisilicide, nickel disilicide, or molybdenumdisilicide. Most frequently, the transition metal silicide is present ina transition metal rich phase which is a solid solution of thetransition metal silicide and elemental silicon.

The transition metal used to provide the transition metal rich phase orthe transition metal silicide rich phase in the alloy may be any metalwhich is either so]- uble in silicon, or in'which silicon is soluble, orwhich forms a silicide in which silicon is soluble. Suitable transitionmetals useful in providing the alloy of this invention include scandium,yttrium, the lanthanides, titanium, lanthanides, hafnium, tantalum,chromium, molybdenum, tungsten, manganese, technetium, rhenium, the irontriad, e.g., iron, cobalt, and nickel, the ruthenium triad, e.g.,ruthenium, rhodium, and palladium, the platinum triad, e.g., osmium,iridium, and platinum; copper, silver, and gold.

The transition metal used to provide the silicide in the alloy isgenerally iron, cobalt, nickel, chromium, manganese, or molybdenum. Mostfrequently iron, co-

balt, or nickel is the transition metal used. Iron is the preferredtransition metal. However, it should be understood that the othertransition metals referred to hereinabove may be used interchangeablytherewith.

The concentration of transition metal should be sufficient to provide asubstantially continuous phase surrounding the regions of the siliconrich phase of the alloy when examined by an optical microscope atmagnifications of greater than about 200 times.

The minimum elemental concentration of transition metal in the alloy isthe amount necessary to give rise' to the second or transition metalrich phase. This will generally be an amount at or just above thesolubility limit of the transition metal silicide in the silicon, i.e.,the concentration at which the transition metal silicide first appearsas a separate phase. For iron, this is approximately 4 weight percentiron; for cobalt, approximately 9 weight percent cobalt; for molybdenum,approximately weight percent molybdenum. For nickel,

' or chromium, or manganese, the amount necessary is at the trace level,e.g., l weight percent or more.

The concentration of the transition metal should be sufficient toprovide a liquidus phase around the solid silicon phase during the earlystages of solidification, thereby relieving the thermal stresses withinthe silicon phase created upon solidification. Preferably, theconcentration of transition metal should be sufficient to provide analloy having a co-efficient of expansion upon solification of less thanabout plus 10 percent, e.g., on the order of about 5 percent or less.

As a practical matter, the concentration of transition metal in thealloy should be sufficient to maintain the corrosion resistance at thelevels provided by the elemental silicon. Accordingly, the silicon richphase should be maintained as the metallographicallypredominant phasewithin the alloy with about twothirds, and preferably three quarters ormore of the silicon being present in the silicon rich phase. When thetransition metal is iron, and the concentrations are in weightpercentsas the elements, the iron content should be less than 39 percent,generally less than about l8 percent, and preferably less than about l4percent. When the transition metal is cobalt, the concentration ofcobalt should be less than about 38 weight percent, generally less than18 weight percent. and preferably less than about 12 weight percent.When the transition metal is nickel, the concentration of nickel shouldbe less than 50 weight percent and preferably less than about 15 weightpercent, and generally less than about 7 weight percent. When thetransition metal is manganese, the concentration of manganese should beless than weight percent, generally less than l5 weight percent, andpreferably less than about 6 weight percent. When the transition seriesmetal is chromium, the concentration of the chromium should be less thanabout 28 weight percent, generally less than about I8 weight percent,and preferably less than about 6 weight percent.

In the alloys herein contemplated, when the transition metal is iron,the concentration of the iron should be from about 4 weight percent toabout I4 weight percent, preferably about 8 weight percent. When thetransition metal is cobalt. the concentration of cobalt should be fromabout 9 weight percent to about l2 weight percent and preferably about10 weight percent. When the transition metal is nickel, theconcentration of nickel should be about I weight percent to about 7weight percent, and preferably about 6 weight percent. When thetransition metal is chromium, the concentration of the chromium shouldbe from about 1 weight percent to about 6 or 7 weight percent andpreferably from about 6 weight percent. When the transition metal ismanganese, the concentration of manganese should be from about I toabout 6 weight, and preferably about 6 weight percent.

When scandium, yttrium, ora lanthanide is the transition metal, thetransition metal, in whatever form it is present, should be from I toabout l8 atomic percent of the alloy. When titanium, zirconium, orhafnium is the transition metal, the transition metal is whatever formit is present, should be from L5 to 7 atomic percent of the alloy. Whenvanadium, columbium, or tantalum is the transition metal, the transitionmetal, in whatever form it is present, should be from about I6 to 1atomic percent. A particularly outstanding alloy is one containing fromabout 1.0 to about 2.0 atomic percent tantalum. When. the transitionmetal is tungsten the tungsten content should be about 1 atomic percentin whatever form the tungsten is present. When the transition metal iscopper, silver, or gold, the transition metal content is from about 1 to20 atomic percent.

The preferred alloys of this invention are three phase alloys,containing a transition metal or transition metal silicide rich phase, adopant or dopant silicide rich phase, and a silicon rich phase. Thesilicon rich phase is the predominant phase and is substantiallydiscontinuous, broken into numerous individual regions of a silicon richphase. In a preferred exemplification of this invention, the transitionmetal rich phase forms narrow rivulets around the boundaries of theregions of the silicon rich phase. Preferably, the rivulets aresubstantially continuous around all of the regions of the silicon richphase. The dopant rich phase is present as nodules at the boundariesbetween the phases and within the individual phases.

The photomicrographs of one particularly preferred exemplification ofthis invention are shown in the Figures. 1

FIG. I is a scanning electron microscope photomicrograph of a polishedsection of an alloy containing 8 percent iron, 0.3 weight percent boron,balance silicon at the magnification shown in the lower left hand cornerthereof.

FIG. 2 is a section of FIG. I at higher magnification.

FIG. 3 is a section of FIG. 2 at higher magnification.

FIG. 4 is a scanning electron microscope photomicrograph of a polishedsection of an alloy containing less than 0.5 weight percent iron, 0.3weight percent boron, balance silicon, at the same degree ofmagnification as FIG. 2. 7

FIG. 5 is a scanning electron microscope photomi crograph of a polishedsection of an alloy containing 30 weight percent iron, 0.5 weightpercent boron, and balance silicon, at the same magnification as FIGS. 2and 4.

As shown in FIGS. 1, 2, and 3, the predominant phase is a discontinuoussilicon rich phase. The phase has well-defined phase boundaries. Theexact composition of the phase is not known, but it is believed to be asolid solution of elemental silicon and an iron silicide containing inexcess of percent silicon, the phase is predominantly silicon with lessthan about 2 percent total silicides, e.g., iron silicide and boronsilicide, therein. Within the discontinuous silicon-rich phase, i.e.,the solid solution and silicides, the silicides are present at or belowthe solubility limit of the silicides in the silicon.

The silicon-rich phase is seen to be surrounded by substantiallycontinuous rivulets of a transition metal rich phase. The rivulets ofthe transition metal rich phase surrounding the silicon rich phase arebelieved to contain iron disilicide, FeSi in the form of a solidsolution of the disilicide and elemental silicon. The elemental siliconis present at or below the solubility limit of elemental silicon in thesilicide. This is the material referred to in the literature aslebeauite.

The rivulets generally are of a width of microns or less, separatingregions of the silicon-rich phase. Frequent pools of 100, 200 or moremicrons in diameter are observed between the silicon rich phases.

Nodules of the dopant rich phase are observed to be present in bothphases and at the boundaries thereof. The nodules are of particularlyhigh melting constituent of the alloy and are silicon-boron phase, e.g.,a boron silicide or silicon boride.

As can further be seen, the silicon rich phase is a predominant althoughdiscontinuous phase containing many regions that are several thousandmicrons or more in their greatest dimension, i.e., in length or indiameter.

ln the alloy having the grain structure shown in FIG. 4, containing lessthan 0.5 weight percent iron, and 0.3 weight percent boron, elementalbasis, the silicon rich phase is not only the predominant phase, but isa substantially continuous phase. The iron silicide phase is both aminor phase and a discontinuous phase.

The alloy having the'grain structure shown in FIG. 2, discussed abovewith reference to FIGS. 1 through 3, is characterized by a predominant,discontinuous silicon rich phase, a minor, continuous iron silicidephase, and nodules of a boride phase.

in the alloy having the grain structure shown in FIG. 5, containing 30weight percent iron, and 0.3 weight percent boron, elemental basis, theiron silicide phases are seen to be continuous. They are also seen to bethe predominant phases, providing an alloy that is predominantly aferro-silicon with only a minor silicon phase.

The following Examples are illustrative.

EXAMPLE 1 To determine the effect of the absence of the dopant, asilicon alloy ingot was prepared containing 8 percent weight iron, 0.3weight percent boron, and balance silicon. The ferro-silicon used inthis test was Ohio Ferro Alloys ferro-silicon having a nominal ironcontent of 35 weight percent and an actual iron content of 30 weightpercent. Seven hundred grams of the ferro-silicon was placed in a No. 10graphite crucible. The crucible was heated to l,580C., for approximately65 minutes. The molten alloy was then poured into a 1 inch by 1 inch by5 inch graphite mold. After the ferro-silicon solidified and cooled, theelectroconductivity of the ingot was measured using a Weston Model 91 1milliameter with power supplied through a Kokour Company siliconrectifier. The electrical conductivity was found to be 32(ohm-centimeter)". The ingot does not crack or develop stress fractures,indicating a coefficient of volumetric expansion upon solidification ofless than 10 percent.-

An alloy was then prepared to test the effect of the dopant, containing8 weight percent iron, 0.3 weight percent boron, balance silicon. Inpreparing this alloy, 12 pounds ferro-silicon, containing weight percentsilicon, 12 pounds of silicon, and 152 grams of fused sodium tetraborate(Na B O were placed in a graphite crucible. The crucible was placed in afurnace and heated to 1,435C. for approximately 50 minutes. Then, themolten iron-boron-silicon alloy was poured into a mold that had beenpre-heated to l,0O0C. After the metal had solidified, and cooled itselectroconductivity was measured using a Weston Model 911 Milliameterwith power supplied through a Kokour Company silicon rectifier. Theelectrical conductivity was found to be 1,090 (ohm-centimeters)". Theingot did not crack or develop stress fractures, indicating acoefficient of volumetric expansion upon solidification of less than 10percent.

Thereafter, the sample was cut, and the uncut surface of the sample wassandblasted and washed with Comet (TM) household cleanser. The samplewas then etched for 5 minutes in a 2.5 normal sodium hydroxide solutionof C., rinsed in water, and airdried.

Two costs of an undercoating solution of 20 grams of EnglehardIndustries ruthenium trichloride containing 39.71 weight percent ofruthenium, elemental basis, in

I 380 grams of US. Industrial Chemical Co. absolute ethyl alcohol wereapplied to the uncut surface of each sample. After each cost, the samplewas heated to 350C. for 15 minutes.

Thereafter three coats of an outer coating solution were applied abovethe undercoating. The outer coating solution was prepared by firstdissolving 54.3 grams of K and K Laboratories titanium chloride in 154.5grams of a 15 weight percent aqueous solution of Fisher ScientificCompany hydrochloric acid. One hundred grams of this composition weremixed with 50 grams of a Mallinckrodt absolute methyl alcohol andsufficient Baker and Adams 30 weight percent hydrogen peroxide to causethe liquid to turn brown. This was then mixed with 60 grams of a liquidcomposition that had been prepared from 15 grams of Englehard Industriesruthenium trichloride and 60 grams of Mallinckrodt absolute methylalcohol. Five coats of the resulting liquid composition were applied tothe previously coated surface of the sample. After each of the coats theelectrode was heated to 350C. for 10 minutes. After the last coat, theelectrode was heated to 450C. for 16 hours. The electrode had a chlorineovervoltage of 0.03 to 0.06 volts at 200 amperes per squre foot in achlorinated brine solution containing 315 grams per liter of sodiumchloride.

EXAMPLE ll lron-boron-silicon alloys were prepared containing 12 percentiron, varying amounts of boron, and the balance silicon.

The ferro-silicon used in this test was Ohio Ferro Alloys ferro-siliconhaving a nominal iron content of 15 weight percent and an actual contentof 12 weight percent.

Seven hundred grams of the ferro-silicon was placed in a No. 10 graphitecrucible. The crucible of ferrosilicon was heated to 1,580C. forapproximately 1 Two electrodes were prepared having bases containing 12weight percent iron, 0.5 weight percent boron, and the balance silicon.In preparing these electrodes, 1,800 grams of ferro-silicon and 42 gramsof fused sodium tetraborate (Na B O were placed in a No. 10 graphitecrucible. The crucible was placed in a furnace and heated to 1,580C. forapproximately 1 hour. Then the molten ferro-silicon, containing boron,was poured into a pre-heated, 3- 4 inch X 1 inch X 6 inch graphite mold.

After the metal had solidified and cooled, two inch X inch X inchsamples were cut from the ingot. The samples showed no signs of crackingor stress fractures, indicating a coefficient of volumetric expansionupon solidification of less than 10 percent. The uncut surface of eachsample was sandblasted and washed with Comet" (TM) household cleanser.Each sample was then etched for 5 minutes in a 2.5 normal sodiumhydroxide solution at 90C., rinsed in water, and air dried.

Three coats of an undercoating solution of 2 grams of EnglehardIndustries ruthenium trichloride in 18 grams of U.S. Industrial Chemicalabsolute ethyl alcohol were applied to the uncut surface of each sample.

which are in the full intended scope of this invention as defined by theappended claims. i

1 claim: V

l. A silicon base alloy having an electroconductivity greater than 100(ohm-centimeters), consisting essentially of silicon,- a dopant, and atransition metal present as the silicide thereof, and comprising:

After each coat,- the sample was heated to 350C. for

10 minutes.

Thereafter three coats of an outer coating solution were applied abovethe undercoating. The outer coating solution was prepared by dissolving18.1 grams of K and K Laboratories titanium chloride in 51.5 grams of a15 weight percent aqueous solution of Fisher Scientific Companyhydrochloric acid. Two grams of this liquid composition were mixed with1 gram of Mallinckrodt absolute methyl alcohol and 0.5 gram of Baker andAdams 30 weight percent hydrogen peroxide. This liquid composition wasthen mixed with 1.2 grams of a liquid compositionthat had been preparedfrom 1 gram of Englehard Industries ruthenium trichloride and 4 grams ofMallinckrodt absolute methyl alcohol. Three coats of this liquidcomposition were applied to the previously undercoated surfaces of eachsample. After each of the first two coats, the electrode was heated to350C. for 10 minutes. After the last coat, each electrode was heated to450C. for 30 minutes.

The resulting electrodes had bulk electroconductivities of 1,500(ohm-centimeters)". Each of the electrodes had 'a chlorine overvoltageof 0.08 to 0.10 volts at 200 amperes per square foot in a chlorinatedsolution containing 3 l 5, grams per liter of sodium chloride.

It is to be understood that although the invention has been describedwith specific reference to details of particular embodiments thereof, itis not to be so limited since changes and alterations therein may bemade a predominant, discontinuous silicon rich phase; discrete nodulesof a dopant rich phase; and substantially continuous rivulets of atransition metal silicide rich phase surrounding the silicon richphaseand forming the boundaries thereof. 2. The alloy of claim 1 wherein thedopant is chosen from the group consisting of boron and phosphorous.

3. The alloy of claim 2 wherein the dopant content is greater than thesolubility of the dopant in silicon.

4. The alloy of claim 2 wherein the dopant content is from 0.2 to2.0weight percent.

5. The alloy of claim 1 wherein the transition metal is chosen from thegroup consisting of iron, cobalt, nickel, chromium, molybdenum andmanganese.

6. The alloy of claim 5 wherein the transition metal content is greaterthan the solubility of the transition metal in silicon.

7. The alloy of claim 5 wherein the transition metal is iron and theiron content is from about 4 to about 14 weight percent.

8. The alloy of claim 1 wherein the alloy has a volumetric coefficientof expansion upon solidification of less than 10 percent.

9. A silicon alloy'consisting essentially of sufficient dopant toprovide an electrical conductivity of greater than about .100(ohm-centimeters), sufficient transition metal silicide to provide anexpansion on solidification of less than 5 percent, and balance silicon.

10. The alloy of claim 9 wherein the dopant content .is chosen from thegroup consisting of boron and phosphorous. I

11. The alloy of claim 10 wherein the dopant content is greater than thesolubility of the dopant is silicon.

12. The alloy of claim 11 wherein the dopant is boron and the dopantcontent is from about 0.1 to about 2.0 weight percent.

13. The alloy of claim 9 wherein the transition metal silicide is chosenfrom thhe group consisting of the silicides of iron, cobalt, nickel,chromium, and manganese.

14. The alloy of claim 13 wherein the content of the transition metalsilicide is greater than the solubility of the transition metal silicidein silicon but insufficient to form a predominant transition metalsilicide phase.

15. The alloy of claim 14 wherein the transition metal is iron and thecontent of the transition metal in the alloy, elemental basis, is fromabout 4 to about 14 weight percent.

16. The alloy of claim 9 wherein the alloy comprises:

phase and forming the boundaries thereof.

1. A SILICON BASE ALLOY HAVING AN ELECTROCONDUCTIVITY GREATER THAN 100(OHM-CENTIMETERS)-1, CONSISTING ESSENTIALLY OF SILICON, A DOPANT, ANDTRANSITION METAL PRESENT AS THE SILICIDE THEREOF, AND COMPRISING: APREDOMINANT, DISCONTINOUS SILICON RICH PAHSE; DISCRETE NODULES OF ADOPANT RICH PHASE; AND SUBSTATIALLY CONTINUOUS RIVULETS OF A TRANSITIONMETAL SILICIDE RICH PAHSE SURROUNDING THE SILICON RICH PHASE AND FORMINGTHE BOUNDARIES THEREOF.
 2. The alloy of claim 1 wherein the dopant ischosen from the group consisting of boron and phosphorous.
 3. The alloyof claim 2 wherein the dopant content is greater than the solubility ofthe dopant in silicon.
 4. The alloy of claim 2 wherein the dopantcontent is from 0.2 to 2.0 weight percent.
 5. The alloy of claim 1wherein the transition metal is chosen from the group consisting ofiron, cobalt, nickel, chromium, molybdenum and manganese.
 6. The alloyof claim 5 wherein the transition metal content is greater than thesolubility of the transition metal in silicon.
 7. The alloy of claim 5wherein the transition metal is iron and the iron content is from about4 to about 14 weight percent.
 8. The alloy of claim 1 wherein the alloyhas a volumetric coefficient of expansion upon solidification of lessthan 10 percent.
 9. A silicon alloy consisting essentially of sufficientdopant to provide an electrical conductivity of greater than about 100(ohm-centimeters) 1, sufficient transition metal silicide to provide anexpansion on solidification of less than 5 percent, and balance silicon.10. The alloy of claim 9 wherein the dopant content is chosen from thegroup consisting of boron and phosphorous.
 11. The alloy of claim 10wherein the dopant content is greater than the solubIlity of the dopantis silicon.
 12. The alloy of claim 11 wherein the dopant is boron andthe dopant content is from about 0.1 to about 2.0 weight percent. 13.The alloy of claim 9 wherein the transition metal silicide is chosenfrom thhe group consisting of the silicides of iron, cobalt, nickel,chromium, and manganese.
 14. The alloy of claim 13 wherein the contentof the transition metal silicide is greater than the solubility of thetransition metal silicide in silicon but insufficient to form apredominant transition metal silicide phase.
 15. The alloy of claim 14wherein the transition metal is iron and the content of the transitionmetal in the alloy, elemental basis, is from about 4 to about 14 weightpercent.
 16. The alloy of claim 9 wherein the alloy comprises: apredominant, discontinuous, silicon rich phase; discrete nodules of adopant rich phase within the boundaries of the silicon rich phase;substantially continuous rivulets of a transition metal silicide richphase surrounding the silicon rich phase and forming the boundariesthereof.